To date, solution polymerization that attains controlled viscosity at certain solids content has not been successfully achieved. Prior art techniques require specially designed equipment to carry out multiple process steps. It would, therefore, be advantageous to develop a polyurethane-forming system having a viscosity that falls within a predetermined range at certain solids content so that it can be successfully employed in biomedical applications, including dipping and infusion.
Segmented copolymers typically derive good mechanical properties from the separation of microphases caused by immiscibility of the segments. For example, it is known that in thermoplastic polyurethane elastomers, the so-called “hard” and “soft” segments have limited miscibility and separate to form microdomains. Many of the properties of polyurethane elastomers can be rationalized in terms of a semi-crystalline hard domain providing a reinforcement or filler-like effect within a soft matrix. The soft matrix or domain, most frequently comprises a poly(alkylene ether) or polyester chain of molecular weight within the range of about 500 to 2000. Such short polymer chains are generally terminated with hydroxyl groups and known as “macrodiols”.
The structure of the macrodiol plays a significant role in determining the performance of the segmented copolymer. Polyester-based macrodiols generally give good mechanical properties, but poor resistance to degradation in harsh environments experienced in for example marine and biomedical applications.
Polyether macrodiols offer enhanced stability, but are not suitable for the synthesis of extremely soft materials, particularly when high stability is also required.
Polysiloxane-based polymers, especially polydimethyl siloxane (PDMS) exhibit characteristics such as low glass transition temperatures, good thermal, oxidative and hydrolytic stabilities and low surface energies. These properties would be desirable in the macrodiol-derived component of a segmented copolymer. In addition, they display good compatibility with biological tissues and fluids and low toxicity. For these reasons, PDMS has found particular application in the construction of medical devices, especially implantable devices. However, polymers derived from PDMS do not generally exhibit good tensile properties such as flexural strength or abrasion resistance.
Considerable efforts have gone into finding a means for incorporating low molecular weight PDMS segments into segmented copolymers such as polyurethanes. These efforts have mainly focused on achieving clarity, processability and a good balance of mechanical properties. However, no completely successful attempts have been disclosed.
As a result of large differences in solubility parameters of PDMS and most conventional hard segments, PDMS-based polyurethanes are likely to be highly phase separated materials characteristic of poor mechanical properties. As a result of this large difference in polarity between hard and soft segments, it is anticipated that premature phase separation occurs during synthesis and there is compositional heterogeneity and overall low molecular weight. In addition, there appears to be an optimal degree of mixing at the interface between soft and hard domains, with extremely sharp interfaces leading to a low degree of mechanical coupling between the two domains and resulting poor strength. Thus it is understood that, for example, PDMS-based polyurethanes generally exhibit poor mechanical properties. Typically, the tensile strength and elongation at break are about 7 MPa and 200%, respectively.
Polycarbonate macrodiols have also been used as reactive ingredients in the synthesis of block and segmented copolymer systems, in particular high performance polyurethanes. Processes for preparing polycarbonate macrodiols based on a range of bishydroxy alkylene compounds are disclosed in JP 62,241,920 (Toa Gosei Chemical Industry Co. Ltd.), JP 64,01,726 (Dainippon Ink and Chemicals, Inc). JP 62,187,725 (Daicel Chemical Industries. Ltd.) DE 3,717,060 (Bayer A. G.), U.S. Pat. No. 4,105,641 (Bayer Aktiengesellschaft), U.S. Pat. No. 4,131,731 (Beatrice Foods Company) and U.S. Pat. No. 5,171,830 (Arco Chemical Technology). The most common alkylenediol described in these patent specifications is 1,6-hexanediol.
Although polycarbonate macrodiols are generally classified under polyesters, the corresponding polyurethanes exhibit hydrolytic stabilities comparable or in some cases superior to polyetherurethanes. They also possess high tensile strength and toughness. These properties are attributed to the high level of phase mixing, promoted by intermolecular hydrogen bonding involving the hard segment urethane hydrogens and the carbonate functional groups of the soft segment. The hydrogen bonding is also partly responsible for the relatively poor elastomeric properties such as low flexibility and high durometer hardness of polyurethanes based on polycarbonate macrodiols. These properties are in contrast to those of the non-polar macrodiol based polyurethanes, such as those based on siloxanes.
A requirement accordingly exists to develop silicon-based macrodiols for use as building blocks of segmented copolymers such as polyurethanes with structural features that exhibit good compatibility and mechanical properties. Suitable macrodiols would retain the advantages of silicon-based polymers such as flexibility, low temperature performance, stability and in some cases biocompatibility. The disadvantages of poor mechanical properties is to be avoided so that the silicon-based macrodiols can form part of materials which can be used in various demanding applications, particularly the biomedical field.